As a device for measuring a property of a fluid, rotational or vibration viscometers or other type of viscometers are conventionally used. Among these, vibration viscometers include a tuning fork vibration viscometer changeable in shear rate (amplitude of a vibrator) (for example, refer to Patent Literature 1).
FIG. 1 is a configuration diagram of a drive mechanical part 2 of a tuning fork vibration viscometer 1, and FIG. 2 is a block diagram of a control system of the tuning fork vibration viscometer of Patent Literature 1. As shown in FIG. 1, the drive mechanical part 2 includes a pair of vibrators 3 and 3 to be dunked in a sample 4, a magnet 10a and an electromagnetic coil 10b for vibrating the vibrators 3 and 3, and a displacement detection sensor 11 for detecting an amplitude of the vibrators 3 and 3. The tuning fork vibration viscometer of Patent Literature 1 includes, as shown in the block diagram of FIG. 2, a sine wave generating circuit 13, a comparator 14, a controller 15, an I/V converter 16, an A/D converter 17, and a microcontroller 18. A PWM modulation circuit 12 for changing a shear rate (amplitude value) is connected to (or incorporated in) the microcontroller 18.
The tuning fork vibration viscometer controls the pair of vibrators 3 and 3 as control objects, and a driving current to be applied to the electromagnetic coil 10b is feedback-controlled (PID control) by the controller 15 so that an amplitude of the vibrators 3 and 3 corresponds to a target amplitude value (target value) that is modulated in pulse width and input from the microcontroller 18. By utilizing a proportional relationship between a driving current to be applied to the electromagnetic coil 10b and a viscosity of the sample 4, the viscosity of the sample 4 in a case where the amplitude changes is measured.